1. Field of the Invention
The present invention relates to a bearing, and more particularly to a combined bearing having advantages of high hardness and self lubrication.
2. Description of Prior Art
A bearing is commonly used to reduce friction in a mechanical system. An oil impregnated bearing, ball bearing, or ceramic bearing is often used in an electric cooling fan. An oil impregnated bearing is commonly made of powdered bronze, an iron-copper based alloy or another metal alloy by a sintering process. The oil impregnated bearing is formed into a desired shape, and has a porous surface in which lubricant oil is impregnated and held by capillary action, in much the same way as water is held in a sponge. When a moving part such as a shaft or a pin moves against the bearing, impregnated oil is forced out and lubricates points of friction. A main advantage of the oil impregnated bearing is its ability to self lubricate over its working lifetime, at least under ideal conditions. However, the oil impregnated bearing is prone to easily wear out due to its low hardness, and may enjoy only a short working lifetime. A ceramic bearing is made of ceramic material, such as silicon nitride or zirconium oxide. The ceramic bearing has a relatively high hardness and a much longer lifetime than the oil impregnated bearing. However, the ceramic bearing has a highly dense surface, in which lubricant oils cannot be impregnated. Thus, great friction exists in the ceramic bearing, which makes it very difficult to start up an electric cooling fan employing the ceramic bearing. Moreover, once the ceramic bearing has been in service for an extended period of time, it is prone to generate much noise. A ball bearing commonly comprises an inner ring, an outer ring and a plurality of balls made of hardened steel or stainless steel between the inner and outer rings. The ball bearing is relatively costly to manufacture. In addition, it is prone to generate much noise once has been in service for an extended period of time.